Conventionally, there is an optical modulator using a pn diode or a pin diode. Such an optical modulator as described includes an optical waveguide core including a pn j unction portion or a pin junction portion, an electrode, and a channel that electrically connects the optical waveguide core and the electrode to each other. The optical modulator modulates light by changing an electric signal to be supplied from the electrode to the optical waveguide core through the channel.
As such an optical modulator as described above, for example, there are a rib waveguide type optical modulator that uses a pn diode, a side-wall grating waveguide type optical modulator that uses a pin diode and so forth.
Of such optical modulators, the rib waveguide type optical modulator that uses a pn diode includes a rib portion that functions as an optical waveguide core and a slab portion that functions as a channel. In the rib waveguide type optical modulator, light is confined in and propagates along the rib portion by a film thickness difference between the rib portion and the slab portion. In particular, in the rib waveguide type optical modulator, the slab portion is formed with a fixed thickness over the entire area thereof in the light propagation direction while the thickness thereof is formed smaller than that of the rib portion so that light is selectively confined in and propagates along the rib portion.
Meanwhile, the side-wall grating waveguide type optical modulator that uses a pin diode includes a grating provided partially along the light propagation direction on a side wall of the optical waveguide core, and the grating functions as a channel. In the side-wall grating waveguide type optical modulator, the region in which the grating is formed exhibits an average refractive index lower than that in the optical waveguide core so that light is confined in and propagates along the optical waveguide core. In other words, in the side-wall grating waveguide type optical modulator, the grating has a thickness equal to that of the optical waveguide core and is formed partially along the light propagation direction so that light is selectively confined in and propagates along the optical waveguide core.